Answer: C. Oak tree.
Explanation: Producers are organisms that are capable of synthesizing their own food using energy from the sun, water, and nutrients from the soil. They form the base of the food chain and provide energy for all other organisms in the ecosystem. In the given food web, the oak tree is the only organism that produces its own food through photosynthesis, making it a producer. The oak borer, mouse, and woodpecker are all consumers that depend on the oak tree or other organisms in the food web for their energy needs.